The increasing sophistication of electronic circuits and systems presents unique challenges for circuit designers. The operating frequency of modern electrical and electronic equipment continues to increase, in order to reduce the physical size and weight of the electronic circuits and systems. However, the trend is hindered by the possible generation of undesirable effects, such as ringing and resonance, due to parasitic effects associated with the components, the physical orientation of the components, and/or the layout of components, devices and/or conductive tracks on printed circuit boards within an electronic circuit or system. These parasitic elements degrade the high-frequency performance of the entire electronic circuit or system.
There are many electronic circuit or system designs where parasitic inductance is a significant element that restricts circuit performance. Given an applied voltage, the parasitic inductance limits the rate at which the current can change. At high frequencies, parasitic inductance can have a major impact on chip performance and can cause chip failure if inductance is not properly detected and corrected. Self inductance is a property of a circuit whereby a change in current causes a change in voltage especially in circuit designs containing long paths or board traces. Mutual inductance comprises the full electromagnetic effect of one current loop over another especially in circuit designs containing long paths that are shielded.
A capacitor is one of the key components in the input and output filters of an electrical circuit. It is typically used as a shunt element to attenuate undesirable signals. However, its equivalent series inductance (ESL) and equivalent series resistance (ESR) significantly affect the capacitor's high frequency (HF) performance, causing non-ideal filter behavior.
FIG. 1 shows the schematic representation of a prior art high-frequency model for a capacitor 10. The capacitor 10 behaves like an inductor when the operating frequency is higher than the damped resonance frequency of the capacitor 10. The damped resonance frequency is determined by the capacitance of the capacitor, its ESL 12, and its ESR 14. FIG. 2 shows an impedance against operating frequency curve 20 for a 470 μF electrolytic capacitor, in which its ESL is 147 nH and its ESR is 67 mΩ. The damped resonance frequency fdr of the capacitor 10 coincides with the minimum value of impedance as illustrated in FIG. 2. The capacitor impedance is dominated by the ESL at high frequencies and its impedance increases with the operating frequency.
The ESL and ESR will introduce undesired voltage ripple at the output of the filter, conducted noise at the input of the filter, and resonance with the other components and parasitic element in the circuit. FIG. 3 shows a buck converter 30 with prior art capacitors 10 as the input and output capacitors. The supply source 32 of the converter 30 is vin. The duty time of the switching element S 34 is adjusted to control the output voltage 36 across the load. FIG. 4 depicts the effects of ESL and ESR on the output voltage of the buck converter 30 when there is a sudden load change. The initial voltage spike 40 is firstly caused by ESL. Then, the effect of the ESR that causes a voltage step 41 that follows. After the transient period settles, the capacitor 10 will discharge to the load 36. Thus, it is crucial to cancel the effects caused by the ESL and ESR.
One prior-art approach for overcoming the parasitic effects is to connect several capacitors of different types in parallel so that different frequency ranges can be covered. However, this only partially resolves the problem at the expense of increasing the physical size, complexity and cost of the electronic circuit or system. Moreover, the added capacitors might resonate with the stray inductance within the circuit and the ESL of other added capacitors.
There are two main approaches to canceling the effects of parasitic inductance on a circuit or system. The first is based on canceling the parasitic inductance of a particular component while the second is based on canceling the effect caused by all parasitic inductances in the entire circuit or system.
As shown in FIG. 5, some coupled magnetic windings 11a, 11b are used to nullify the effect of the parasitic inductance of the capacitor 10. The coupled windings 11a, 11b will give an equivalent negative inductance in series with the capacitor. Although the ESL can be canceled, the structure will produce an additional inductance in series with the load 36. Moreover, it cannot cancel the ESR effect. This will affect the dynamic behavior of the output voltage across the load 36. Such structure is more suitable for input filter design. Nevertheless, the coupling effect is highly dependent on the magnetic properties of the core materials of the coupled windings.
A prior-art method using the second approach is based on adding passive circuits. The parasitic effects are canceled at the circuit level. By extending the first approach, a coupled winding that can cancel parasitic inductance of capacitors in an electromagnetic interference (EMI) filter has been proposed. In other approaches, some active noise cancellation circuits that can cancel the undesired effects caused by the parasitic elements have been proposed.